Plasma discharges can be used to dissociate gases to produce activated gases containing ions, free radicals, atoms and molecules. Activated gases are used for numerous industrial and scientific applications including processing materials such as semiconductor wafers, powders, and other gases. The parameters of the plasma and the conditions of the exposure of the plasma to the material being processed vary widely depending on the application.
For example, some applications require the use of ions with low kinetic energy (i.e. a few electron volts) because the material being processed is sensitive to damage, or because there is a requirement for selective etching of one material relative to another. Other applications, such as anisotropic etching or planarized dielectric deposition, require the use of ions with high kinetic energy.
Some applications require direct exposure of the material being processed to a high density plasma. Such applications include ion-activated chemical reactions and etching of and depositing of material into high aspect-ratio structures. Other applications require shielding the material being processed from the plasma because the material is sensitive to damage caused by ions or because the process has high selectivity requirements.
Plasmas can be generated in various ways including direct current (DC) discharge, radio frequency (RF) discharge, and microwave discharge. DC discharges are achieved by applying a potential between two electrodes in a gas. RF discharges are achieved either by capacitively or inductively coupling energy from a power supply into a plasma.
Parallel plates can be used to capacitively couple energy into a plasma. Induction coils can be used to induce current in the plasma. Microwave discharges can be produced by coupling a microwave energy source to a discharge chamber containing a gas.
Plasma discharges may be generated in a manner such that both the charged species constituting the plasma and the neutral species, which may be activated by the plasma, are in intimate contact with the material being processed. Alternatively, the plasma discharge may be generated remotely from the material being processed, so that relatively few of the charged species come into contact with the material being processed, while the neutral species may still contact it.
Such a plasma discharge is commonly termed a remote or downstream plasma discharge. Depending on its construction, positioning relative to the material being processed, and operating conditions (gas species, pressure, flow rate, and power into the plasma), a plasma source can have characteristics of either or both of these two general types.
Existing remote plasma sources generally utilize RF or microwave power to generate the plasma. Although present sources support many applications successfully, several technical limitations remain in the practical use of those sources.
Microwave-based remote plasma sources are generally more expensive than RF sources because microwave power is generally more expensive to produce, deliver, and match to a load. Microwave sources and a power delivery system are also generally more bulky than RF sources and require periodic replacement of a tube which generates the microwave power.
RF remote plasma sources that have some degree of capacitive as well as inductive coupling may be less expensive and smaller than the corresponding microwave sources. The capacitive coupling, however, which assists in the plasma ignition process, also may lead to degradation of the exposed walls of the plasma vessel due to bombardment of those walls by energetic ions produced in the plasma. RF remote plasma sources that utilize inductive RF coupling, but which minimize associated capacitive coupling, may show less ion-induced degradation of the plasma vessel surfaces. The reduction or elimination of the capacitive coupling, however, can make plasma ignition more difficult to obtain, especially over a wide range of process conditions.
A second difficulty with existing remote plasma sources is removal of the heat generated in the plasma and deposited onto the walls of the plasma vessel. This is especially the case when the plasma vessel has a complex shape and when it is composed of a dielectric material for which direct cooling with large quantities of fluid in contact with the dielectric vessel is either undesirable or impractical. This has the effect of limiting the power that can be reliably coupled into the plasma.
Existing toroidal plasma systems, for example, can be highly inductive in the manner in which the RF energy is coupled into the plasma. The plasma may be ignited, for example, via a capacitively coupled RF ignition discharge or via ultraviolet radiation. The plasma system can require separate plasma ignition steps with specific gas species, pressure and flow rate requirements. The specific requirements can be different from operating condition requirements. These constraints can both add additional complexity to the elements of the vacuum and gas handling systems used in conjunction with the plasma system and can also increase the overall time need for processing.